1. Field of the Invention
The present invention relates to a method of and an apparatus for bending and tempering a sheet of glass, especially for use as an automobile window glass sheet, at one stage.
2. Description of the Relevant Art
One general process for tempering a sheet of glass employs air jets which are applied to the glass sheet over its surface. Other glass sheet tempering methods include a process in which a sheet of glass to be tempered is submerged in a liquid, and a process in which a solid contact member made of a material of good thermal conductivity, such as boron nitride, stainless steel, or the like, is held against a sheet of glass to deprive the contacted glass surface of heat.
Japanese Patent Publication No. 62(1987)-40298, for example, discloses an air jet process. According to the disclosed process, a sheet of glass to be tempered is not constrained after it is bent until it is rapidly cooled, and hence the glass sheet tends to be deformed. When a glass sheet is bent to a greater curvature, it is more liable to get deformed.
The solid member contact method is disclosed in Japanese Patent Publications Nos. 62(1987)-18488 and 63(1988)-43324 and Japanese Laid-Open Patent Publications Nos. 59(1984)-213635 and 63(1988)-260833, for example. According to the method disclosed in Japanese Patent Publication No. 62(1987)-18488, a metallic cooling plate for contacting a sheet of glass is divided or has a groove to reduce any thermal expansion thereof due to a temperature rise. The process disclosed in Japanese Patent Publication No. 63(1988)-43324 employs a main cooling device for cooling a sheet of glass with air jets after the glass sheet is preliminarily cooled by contact with a solid contact member.
The method shown in Japanese Patent Publication No. 62(1987)-18488 is disadvantageous in that the glass sheet cannot uniformly be cooled, a drawback which is inherent with the solid member contact process. According to the process shown in Japanese Patent Publication No. 63(1988)-43324, since the glass sheet is cooled in two steps, the overall apparatus required to carry out the method is large in size, and when the glass sheet is bent, it tends to be deformed while being fed after it is preliminarily cooled.
In order to deprive a glass sheet of heat quickly in the solid member contact method, it is necessary to press the solid contact member against the glass sheet relatively strongly. When the glass sheet which has been heated nearly to its softening point is cooled, the glass sheet shrinks to a certain degree. Therefore, if the glass sheet remains to be gripped under pressure by the solid contact member, tensile stresses are developed in the glass sheet owing to the shrinkage caused when the glass sheet is cooled. If the tensile stresses exceed the mechanical strength of the glass sheet, then the glass sheet will crack.